1. Field of the Invention
The invention relates to electrically heated screens, and particularly to rail members which extend longitudinally along the edges of the screen for mounting and tensioning the screen on a pair of spaced side frame members. More particularly, the invention relates to a tensioning rail having a wedge-shaped clamping bar which securely clamps the screen edge to the rail and which provides an improved electrical connection between the rail and screen.
2. Description of the Prior Art
Screen assemblies have been used in industry for many years for screening of materials. The screens may be oscillated mechanically to assist in screening the material, or may be stationary. In either case the screen is heated electrically to reduce binding and clogging of the screen openings. Examples of such electrically heated screen constructions and the means for electrically heating the same are shown in my previously issued U.S. Pat. Nos. 2,704,155, 2,812,062, 2,825,461, 3,154,757, and 3,195,725.
Screen constructions of the type shown in my above-identified patents and in other screen assemblies have rails which extend longitudinally along the edges of the screen or screen panels for mounting and tensioning the screen cloth or fabric on a pair of associated side frame members. These tensioning rails generally are formed of metal having good electrical conducting characteristics and are connected to a source of electrical power which is transferred through the rails to the metal screen material for heating the same. The rails, in turn, are mounted on the side frame members and are electrically insulated therefrom by various insulating means.
The screen edges are connected by various means to the tensioning rails in order to provide an electrical connection between the rail and screen and to provide means for tensioning the screen between the spaced side frame members. Various types of connections have been devised for both electrically heated screen constructions and for mechanically oscillated screen constructions.
One type of screen connection consists of a pair of mutually engaged, reversely angled brackets, such as shown in U.S. Pat. Nos. 1,353,549 and 2,052,467. Another type of connection consists of forming a reversely angled, stiffened and reinforced edge on the screen which engages a horizontal flange on the frame members, such as shown in U.S. Pat. Nos. 1,663,164 and 1,906,336. Other screen tensioning devices use a V-bar which clamps the screen portions adjacent its edges in a V-grooved channel, such as shown in U.S. Pat. Nos. 2,213,773, 2,378,463, and 3,776,382.
One of the most common types of screen tensioning means which is used, particularly in electrically heated screen assemblies, consists of angled blocks or flanges mounted on the frame members about which reinforced hook-shaped, stiffened screen edges are engaged. An example of this type of connection is shown in my heated screen assembly of U.S. Pat. No. 3,195,725.
Problems have been encountered in the past with many of these known tensioning rails, especially tensioning rails using a hook-shaped member which is engaged by complementary reinforced hook-shaped screen edges. It has been found that after extended periods of heated operation of the tensioned wire screen cloth that the heating of the screen results in some expansion and that the screen loosens from its hooked engagement with the flange of the tensioning rail. This results in poor tensioning contact which causes the screen to whip back and forth resulting in premature failure at the point of contact with the tensioning rail. This is a problem in both electrically heated and nonheated screen assemblies. This premature loosening of the screen rail engagement, however, is more critical in electrically heated screens in that it results in poor electrical contact between the screen and rails causing arcing, power loss and inefficient power use.
Prior tensioning rails are formed of relatively thin sheet metal which has a relatively short life when used in installations encountering abrasive materials such as slag, limestone, iron ore, silica sand, etc. These prior tensioning rail constructions are mounted on their side frame members by bolts, the heads of which project outwardly beyond the outer surface of the rails. These bolt heads also are subjected to the abrasive action of the materials being screened thereby.
It has been found that in many electrically heated screen assemblies metal strip edge stiffeners which are formed about and engaged with the edges of the screen, are required to provide the necessary stiffness to the screen. This reinforcing strip enables the screen to be drawn up tightly and tensioned by the rail and provides the necessary electrical contact between the rail and screen when initially installed. These stiffening strips usually are formed of stainless, plain or galvanized steel, thereby increasing both the material and installation costs of a screen assembly. However, screening with stiffening strip edges when heated are subject to the described expansion loosening.
No tensioning rail construction of which I am aware has eliminated these problems by providing a wedge-shaped clamping bar and complementary-shaped channel for firmly mounting the screen thereon.